1. Field of the Invention
The present invention relates to communications systems, in particular radio frequency systems for telemetry, identification, or positioning.
2. Description of the Related Art
The latest advances in wireless communications technologies are mostly in the field of broadband networks. Contrary to this trend, there is a wide range of applications where small to medium amounts of information have to be occasionally sent between participants spread over wide areas. Usually such devices are required to have a small profile and long operational range, to be autonomous, battery powered, and to operate in power saving mode, where reception periods alternate with long periods of power-off, in order to achieve a maintenance-free life of several years. This is the case with conventional applications in telemetry, radio frequency identification (RFID), and location/tracking. Other applications include location-aware computing devices, and data sensor networks. The harsh operational environments of such devices present serious drawbacks to achieving these requirements. For example, some aggravating conditions that impede the use of such devices are:    Limited battery life or need for external power;    External limitations on transmission power, as in operations in hospitals or munitions proximity;    Operation in urban non-line-of-sight (NLOS) situations, as discussed in references [1] to [8], for example;    High levels of interference;    External limitations on the antenna, which typically has to be small, possibly even concealed, and therefore has low gain.
The basics of telecommunications theory show that the rate of information transmitted through a system comes at the expense of the transmission power or the operating distance. So if one lowers the rate of the transmitted information, then the transmitted power can be lowered proportionally. On the other hand, if the power is kept constant then the transmission will reach a longer distance.
Low power itself has a further drawback: as the noise level in the receiver is high compared to the signal level, coherent detection systems are typically required to receive such signals. Other impairments include but are not limited to multipath and Doppler effects. Conventional coherent detectors are typically complex, expensive, and slow to acquire signals.
What is needed is a communications system that enables fast acquisition and coherent detection of low data rate identification signals without the cost and complexity of typical coherent receivers.